1. Field of the Invention
The invention relates generally to hip prosthesis assemblies for replacing a natural hip socket; the form and composition of locking mechanisms used in such assemblies to secure the attachment of assembly components; and to processes for fabricating specific types of locking mechanisms including retaining rings per se, contemplated by the invention.
More particularly, a first aspect of the present invention relates to a prosthetic acetabular cup assembly for receiving a ball attached to a femur. The components of the assembly (an insert bearing component and an outer shell component, both to be described in greater detail hereinafter) are, according to a preferred embodiment of the invention, interlocked via a locking mechanism that includes a retaining ring fabricated at least in part using polyaryletherketone material, such as PEEK.
The locking mechanism used in such assemblies may take any one of a number of shapes that are useful for securing assembly components. Constraints influencing locking mechanism design include insuring that substantially all motion is eliminated between assembled parts, insuring further that push-in/pull-out forces of assembly are within generally accepted industry standards, etc.
A further aspect of the invention is directed to the form and composition of the locking mechanism per se. More particularly, various specific locking mechanisms fabricated using polyaryletherketone materials have been identified which have been found to have advantageous shapes for meeting the aforementioned constraints For example, according to one embodiment of the invention the locking mechanism includes a hex-shaped retaining ring having an inside radial chamfer, where the retaining ring is a composite fabricated at least in part using PEEK combined with a reinforcing material, e.g., carbon fibers with the reinforcing material being added to the PEEK matrix in order to reinforce the composite ring (e.g., to strengthen and add rigidity to the ring).
Still further aspects of the invention are, as indicated hereinabove, directed to processes for fabricating specific types of retaining rings for use in securing the components of an acetabular cup assembly; and to locking rings per se made utilizing polyaryletherketone material.
2. Description of the Related Art
It is known to provide an acetabular cup assembly, including a metal shell component for attachment to an acetabulum, to replace the natural socket; and to provide a polymer bearing component which is inserted into the shell to provide a hemispherical bearing surface for receiving a femur ball prosthesis element. Often, the polymer bearing component (also referred to herein as an "insert" component) is nonsymmetrical and includes a built up lip around a portion of the hemispherical bearing surface to help prevent dislocation of an installed femur ball from the hemispherical bearing surface.
During installation of the acetabular cup assembly, the shell component is first secured to the acetabulum. When a surgeon installs the bearing component, the surgeon selects an orientation of the bearing with respect to the shell component to align the lip of the nonsymmetrical bearing component in the most advantageous position to reduce the likelihood of dislocation of the femur ball.
Therefore, it is desirable to produce an acetabular cup assembly in which (a) the bearing component can be easily attached to the shell component in a large number of selected orientations to provide the maximum degree of flexibility for the surgeon using only a push-in force within some predefined range; and (b) the retention mechanism (for interlocking the shell and insert, also referred to herein as the "locking mechanism") allows the bearing component to be easily oriented and easily installed (as indicated hereinbefore); and easily removed by the surgeon, if necessary, using only a pull-out force within some predefined range.
The installed bearing component must also be secured to the shell component by a retention force (and associated means for inducing such force) that is strong enough to prevent rotation or dislocation of the bearing component from the shell component when the cup assembly is completed and being used as intended as a hip prosthesis device, i.e., after having been installed by the surgeon within the shell at the desired orientation.
An example of prior art addressing some of the aforementioned desirable aspects of an acetabular cup assembly and locking mechanism therefore, is U.S. Pat. No. 5,049,158, issued on Sep. 17, 1991, to Engelhardt et al. U.S. Pat. No. 5,049,158 is hereby incorporated by reference.
The objectives of the incorporated reference (many of which are shared by the present invention) included (a) providing a retention mechanism for retaining a bearing component situated at any selected orientation inside a shell component which does not rely on the physical properties of the bearing component; (b) reducing the loading of forces on an outer lip or flange of the bearing component after the hip prosthesis is installed in a patient; (c) preventing rotation of the bearing component relative to the shell component after insertion of the bearing component into the shell component in a desired orientation; and (d) providing a self adjusting locking mechanism which retains the bearing component inside the shell component despite possible shrinkage of the bearing component after installation.
The incorporated reference met these objectives via its teaching of a prosthetic acetabular cup assembly that includes a single piece bearing component having an inner bearing surface for receiving a ball attached to a femoral prosthesis and an outer surface. The assembly includes a shell component for attachment to an acetabulum to replace a natural hip socket and has an inner surface defining a cavity for receiving the bearing component therein.
According to the incorporated reference, a formed wire is situated in an arcuate groove that is part of the shell component. The formed wire is configured so that a portion of the wire extends radially inwardly from the arcuate groove of the shell component to engage a corresponding, axially aligned, arcuate groove formed in the bearing component to retain the bearing component inside the shell component without the use of attachment screws or the like.
Furthermore, according to the incorporated reference, anti-rotation lugs are formed on the inner surface of the shell component to cut into the outer surface of the bearing component as the bearing component is inserted into the shell component to prevent rotation of the bearing component relative to the shell component.
The incorporated reference goes on to teach the use, according to a preferred embodiment of the invention taught, of a serpentine shaped lock wire to interconnect and lock together the aforementioned bearing and shell components; with the lock wire preferably being made of cobalt chrome material that is shaped by conventional wire forming techniques. The resulting metal wire is, according to the reference, optionally heat treated to increase its strength. Titanium is also suggested by Engelhardt et al. as an alternate metal for fabricating the lock wire.
U.S. Pat. No. 4,380,090, issued Apr. 29, 1983, to Ramos, is another example of prior art which teaches the use of locking rings for an acetabular cup assembly. In particular, Ramos teaches that the lock ring for an acetabular cup assembly is "preferably made of a resistant metal such as Vitallium or stainless steel".
Using metal wire rings as part of a locking mechanism for interlocking components of an acetabular cup assembly, as taught in the prior art exemplified by the patents referred to hereinabove, is inherently problematic. For example, as those skilled in the art will readily appreciate, if the rings are made too thick they are stiff and not easy to work with; if the rings are made to thin they are "sloppy", that is they do not always retain their intended shape, spring capacity, etc.
Further problems with locking mechanisms for acetabular cup assemblies that employ metallic retaining rings include the prospect of unacceptably high push-in and/or pull-out forces being required when respectively assembling or trying to purposely disengage assembly components. For example, in the experiments to be described hereinafter it was found that a pull out force in excess of 1,500 pounds was required for Vitallium (.TM.) rings of approx. 0.050" in diameter.
Further yet, use of metallic wire lock rings, such as the Vitallium rings preferred by the aforementioned Ramos reference, etch, are known to be problematic because of certain inherently difficult and/or expensive process steps required when working with such material. Examples of such process steps include those steps needed to perform the inherently difficult task of converting straight wire into a desired wavy structure; steps needed to heat treat metallic wire retaining rings to increase their strength, etc.
Alternative materials for making lock rings, such as silicone and Ultra-High Molecular Weight Polyethylene (UHMWPE), are also known in the art. However, many of these materials suffer from problems of their own. For example, both silicone and UHMWPE have inconsistent problems with a wide spread of push-in and push-out strengths, problems related to the shell/insert interface being too loose when these materials are used to fabricate the retaining ring, etc. Several of these problems were demonstrated in experiments referred to hereinafter, in the Detailed Description of the invention, where polyethylene was used to fabricate experimental retaining rings.
For all of the reasons set forth hereinabove, it would be desirable to provide an acetabular cup assembly design which is easy to use by the surgeon; is simple to put together and take apart as needed; and is made up of components that are easy to fabricate and replicate in mass at low cost.
As indicated hereinbefore, this desire includes being able to provide: (a) an acetabular cup assembly in which the bearing component can be easily attached to the shell component in a large number of selected orientations to provide the maximum degree of flexibility for the surgeon using only a push-in force within some predefined range; (b) a locking mechanism for use in such assemblies (for interlocking the shell and insert), which allows the bearing component to be easily oriented, easily installed and be easily removed by the surgeon, if necessary, using only a pull-out force within some predefined range; and (c) an acetabular cup assembly in which the bearing component can be secured to the shell component by a retention force (and associated means for inducing such force) that is strong enough to prevent rotation or dislocation of the bearing component from the shell component when the cup assembly is completed (after insertion of the bearing component into the shell component in a desired orientation) and being used as intended as a hip prosthesis device.
Furthermore, in view of the prior art discussed hereinabove, it would be particularly desirable to provide a locking mechanism for an acetabular cup assembly which includes a retaining ring that is made from a material that is strong, easy to machine and does not require the performance of the aforementioned wire shaping and/or heat treatment ring fabrication process steps.
Further yet, it is desirable to provide a locking mechanism for an acetabular cup assembly, that includes a retaining ring that is made from a material that allows the rings to be mass produced by, for examples an injection molding process to realize objectives of being able to minimize locking mechanism fabrication costs, assure consistency in the locking mechanism fabrication process and assure the quality of the components produced by such processes, etc.
Further still, it would be desirable to provide an acetabular cup assembly and a locking mechanism therefore, for use in such assembly to retain the bearing component inside the shell component after installation, which includes a retaining ring that is formed using materials that exhibit consistent push-in and pull-out forces; and result in assemblies that do not experience shell/insert toggle.
In view of the teachings of the incorporated Engelhardt et al. patent, which illustrates the present state of the art, it would be desirable to provide a locking mechanism that prevents rotation of assembly components and substantially eliminates motion between assembly components without having to machine or otherwise form separate means (like Engelhardt's lugs) to prevent such rotation and/or other motion.
Finally, it would be desirable to provide processes per se for fabricating the aforementioned desirable locking mechanisms and retaining rings, particularly those locking mechanisms and retaining rings that are suitable for incorporation into acetabular cup assemblies.